1. Field of the Invention
The present invention relates to an organic electro-luminescence device, and more particularly, to an organic electro-luminescence device capable of improving image quality and a method of driving the same.
2. Background of the Related Art
There are various kinds of flat panel displays that have been recently developed to substitute heavy and bulky cathode ray tubes (CRTs). Examples of these flat panel displays are a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an electro-luminescence display (ELD), etc. Much effort has been made to improve image quality and enlarge screen sizes of the flat panel displays. Among those flat panel displays, the organic electro-luminescence device has attracted considerable attention as a self-luminous display device that emits light by itself. The organic electro-luminescence device displays a predetermined image by recombining electrons and holes. Moreover, the organic electro-luminescence device is driven at a low direct current (DC) voltage and has a high response speed.
FIG. 1 is a circuit diagram illustrating a unit pixel according to the related art. One pixel in the organic electro-luminescence device with N×M pixels is illustrated as representative. As shown in FIG. 1, a current drive transistor M2 is connected to an organic light emitting diode (OLED) so that the current drive transistor M2 supplies a drive current to the OLED to emit light. The amount of the drive current from the current drive transistor M2 is determined by a data voltage Vdata applied through a switching transistor M1. The switching transistor M1 has its gate, source and drain connected to a select signal line 2, a data line 4 and the current drive transistor M2, respectively.
A capacitor Cst is connected between a source and a gate of the current drive transistor M2 to maintain the applied data voltage Vdata during a predetermined interval. Upon operations of pixels, the switching transistor M1 is turned on by a select signal from the select signal line 2, and then the data voltage Vdata is applied to a gate of the current drive transistor M2 through the data line 4. The OLED emits light when receiving a drive current generated according to the data voltage Vdata applied to the gate of the current drive transistor M2. The drive current flowing through the OLED is expressed in an Equation 1.
                              (                      I            OLED                    )                =                                            β              2                        ⁢                                          (                                                      V                    dd                                    -                                      V                    data                                    -                                                                                V                      th                                                                                          )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    gs                                    -                                      V                    th                                                  )                            2                                                          (                  Equation          ⁢                                          ⁢          1                )            wherein IOLED represents the drive current flowing through the OLED, Vdd represents the drive voltage, and Vgs, Vth, Vdata and β represent a voltage between the source and the gate of the current drive transistor M2, a threshold voltage of the current drive transistor M2, a data voltage, and a constant value, respectively.
As illustrated in Equation 1, the drive current IOLED is supplied to the OLED according to the data voltage Vdata, and brightness of the OLED is determined by an intensity of the applied drive current IOLED. Meanwhile, the drive voltage Vdd is supplied through a power voltage supply line 6. The power voltage supply line 6 is connected to the current drive transistor M2. The power voltage supply lines may be formed in horizontal lines and vertical lines in order to be disposed on all pixels of the organic electro-luminescence device. Herein, when the drive voltage Vdd is supplied through an original line resistance in the power voltage supply line 6, a drop voltage is generated as the drive voltage Vdd flows through the power voltage supply line 6. Accordingly, as illustrated in FIG. 2, the power voltage supply line 6 in the left side of the organic electro-luminescence device is disposed far from a power voltage supply line 6′ in the right side of the organic electro-luminescence device, thereby generating a difference of the drive voltage therebetween. That is, the drive voltage Vdd flowing in the right power voltage supply line 6′ is smaller than that supplied to the left power voltage supply line 6. Accordingly, as illustrated in Equation 1, the drive voltage of each pixel depends on the data voltage Vdata when the drive voltage Vdd has a fixed constant value. In this case, brightness of the OLED in each pixel is controlled by a change in the data voltage Vdata.
However, as described above, when the constant data voltage Vdata is supplied to all pixels to obtain an identical gray scale, the drive current in each pixel becomes different because the drive voltage Vdd supplied to each pixel becomes different. Accordingly, a constant gray scale cannot be obtained due to the occurrence of a brightness difference between the pixels. On the other hand, although the voltage Vgs between the source and the gate of the current drive transistor M2 is identical, a deviation is generated in the threshold voltage Vth due to a non-uniformity of manufacturing processes. Thus, the amount of current supplied to the OLED becomes different, thereby causing different luminous brightness. Moreover, since a unit pixel with the same structure is formed on the entire region of the panel, characteristics of the current drive transistor M2 fluctuate according to pixel positions. As a result, there is a limitation in an optimized gray scale realization due to a brightness difference between the pixels.
There is an advantage in that the current drive pixel structure can compensate for the characteristic fluctuations of the device, and a voltage drop of the power voltage supply line. However, when resistance and capacitance of the data line increases, it is difficult to display a low gray level due to charging and discharging of the data line.